1. Field of the Invention
This invention relates to arrangements for extracting charged particles which have been emitted from a sample. Such an arrangement is used, for example, in a mass spectrometry system for transferring ions from a solid or liquid sample, which has been ablated by means of the impact of a primary beam of ions, fast atoms or neutrals, laser radiation or a microscopic localised discharge, to the analyser of the mass spectrometry system. The ions from the sample, so called secondary ions, may be produced directly in the ablation process, or may be produced by subsequent ionization of the ablated material. For double focussing magnetic sector mass spectrometers (DFMSs) or time of flight mass spectrometers (TOFs) it is necessary for the secondary ions to have energies typically in the range 5 to 10 keV in order to be analysed. Thus the secondary ions are normally strongly accelerated as, or soon after, they leave the sample in order to minimise the effects of their intrinsic energy spread on the spatial and mass resolution of the spectrometer.
2. Description of the Prior Art
Recent developments in material science, particularly in semi-conductor device fabrication, high temperature superconductor fabrication and similar areas, has revealed inadequacies in such arrangements as are currently employed. In the analysis of multi-layer solids it is necessary to achieve a high depth resolution together with a simultaneous high sensitivity. Where the particles are produced by a sputtering process induced by a primary ion or neutral beam, that is the technique known as secondary ion mass spectrometry (SIMS), in order to achieve a high depth resolution it is necessary to reduce the impact energy of the primary beam in the direction normal to the surface of the sample, and to ensure that the eroded region of the sample remains flat. Where the particle extraction arrangement includes means for producing a high electrostatic extraction field which is polarised so as to facilitate the extraction of positively charged secondary ions from the sample, if the primary ion beam is positively charged the beam will be retarded and deflected by the extraction field such that the beam must have a certain impact energy in order to reach the sample. Furthermore, where the primary ion beam is scanned over the sample a high extraction field will cause the impact angle of the primary ions to vary across the sample giving rise to local variations in sputter rate which in turn will cause local variations in the density of the secondary ions emitted from the sample, this leading to non-planar erosion of the sample.
Alternatively, where the primary ion beam comprises negatively charged ions, with known arrangements it is not possible to use impact energies of the primary ion beam on the sample of much less than 10 keV, this leading to a consequent degradation in depth resolution. In the presence of a high extraction field which is polarised to facilitate the extraction of positively charged secondary ions from the sample, such a negatively charged primary beam will also suffer the consequences described above in relation to a positively charged primary beam.
A further problem with known arrangements for the extraction of charged particles is that the detection sensitivity of the mass spectrometer for some components within the sample, may be limited by background signals arising from the return of previously emitted particles from the sample which have coated the electrodes surrounding the sample area, and the adsorption of previously emitted particles contained in the residual gas of the vacuum system. Thus, it is essential to maintain a good vacuum in the region immediately surrounding the sample region. This is made difficult, however, by the need in existing arrangements for extracting particles as used in double focussing magnetic sector mass spectrometer systems, to have electrodes close to the sample in order to provide the necessary electrostatic extraction field. This reduces the efficiency with which the sample region may be pumped, thus increasing the probability that emitted particles may be transferred from the electrodes and residual gas back on to the sample.
A further shortcoming with known arrangements for extracting charged particles occurs with the analysis of topographically patterned or rough samples in arrangements having high electrostatic extraction fields, as departure from sample planarity will cause local perturbations of the extraction field. This will cause ions sputtered from the sample to be emitted on divergent trajectories, with the possibility that the secondary ions do not enter the analyser for the mass spectrometer. Where the spectrometer is operating in a microscope mode, the spectrometer will suffer the disadvantage of a small depth focus, this being a particular problem in the analysis of fully or partly fabricated semi-conductor devices.
Known arrangements for extracting charged particles also suffer further disadvantages when used in the analysis of insulating samples, as charging caused by a charged primary ion beam, together with the ejection of charge in the form of emitted charged particles from the sample, will lead to a progressive change in the sample surface potential until the primary beam is deflected away from the sample. Where the primary ion beam contains positive ions, with the extraction field being polarised so as to facilitate the extraction of positive secondary ions from the sample, charging of the sample may be alleviated by simultaneous bombardment of the sample with a beam of electrons of suitable energy so as to produce a gross charge compensation. In known arrangements for extracting charged particles in which the electrostatic extraction field is high the use of an electron beam in order to perform such charge compensation is ineffective as secondary electrons are readily lost from the sample.
A further disadvantage of known arrangements is that an apparatus such as a mass spectrometer having a finite entrance aperture collects ions most efficiently from a small sample area when, by use of optics providing linear magnification of the secondary ion column ejected from the sample, ions initially ejected from the sample into a large solid angle may be accepted by the spectrometer because of the accompanying angular demagnification. In order to collect ions from the sample efficiently from over a large sample area it is necessary to provide means of optimising the spectrometer to a small static field of view and sweeping this field of view over a desired sample area by means of appropriate optical devices. By the use of beam scanning plates or a so-called double deflection scanning system, fields of view of up to 0.5 mm diameter may be achieved if a high electrostatic extraction field is combined with a small extraction gap. However, when analysing very thin layers or attempting to examine both lateral and depth distributions of a sample simultaneously it is necessary to address a sample area of up to 2 mm sq. This is not possible with present arrangements for extracting charged particles without the use of a sample scanning stage.
A further problem arises in the analysis of liquids and liquid suspensions. In the presence of a high electrostatic extraction field, the surface of the liquid may become disrupted causing serious instabilities in particles emitted from the sample. Furthermore, the necessary closely coupled extraction optics will reduce the pumping capability of the vacuum system in the region surrounding the sample. High gas pressures in the region around the sample will inhibit the transmission of ions from the sample and may give rise to destructive arcs between the electrodes in the apparatus for extracting the particles.